1. Field of the Invention
This invention relates generally to magnetic recording hard disk drives (HDDs), and more particularly to a HDD that counts the number of writes to the data tracks to minimize the effect of far track erasure (FTE) during writing.
2. Description of the Related Art
HDDs with high data density are required to have high data track density, which means that the concentric data tracks are packed closer together. High track density increases the problem of adjacent track encroachment, also called adjacent track erasure (ATE). The write field from the write head is generally wider than a data track so when the write head is writing to a track, the outer portions of the write field (called the fringe field) overlap onto tracks adjacent to the track being written. This overlap is the ATE that will result in added noise and degradation of the data on the adjacent tracks. ATE occurs when old data stored in tracks adjacent to the track being written become degraded after many writings to the data track. ATE generally translates into an increase in bit error rate (BER), resulting in degradation of the performance and reliability of the HDD.
One approach for addressing the ATE problem is to count the number of writes to each track. When a track has been written a predetermined number of times, that track and one or more adjacent tracks on both sides of it are read and then rewritten. The effects of ATE cannot build up because significant repetitive writing of a single track is avoided. This approach is described in U.S. Pat. No. 7,567,400 B2, which is assigned to the same assignee as this application.
However, it has been discovered that data degradation due to fringe fields is not limited to adjacent tracks, but can extend over a range of tracks relatively far from the track being written. This effect is sometimes called wide-area track erasure (WATER) or far track encroachment or erasure (FTE). FTE is particularly noticeable with perpendicular write heads that have side shields. FTE may not affect tracks symmetrically on both sides of the track being written. Tracks on one side may encounter more pronounced FTE effects due to the write head shield design or due to read-write head skew. FTE is described by Liu et al., “Characterization of Skip or Far Track Erasure in a Side Shield Design”, IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009, pp. 3660-3663.
Thus what is needed is a HDD that counts the writes to the data tracks in a manner that effectively addresses the problem of FTE.